You will want the f2 if you can swing it. You are already losing a decent amount of light on the Redrock and if you need to do any shooting indoors, you will be happy that you can open up to f2.
As far as those other zooms go, I like the Sigma. If you can find one, Sigma made a 24-60 f2.8 and it was pretty sweet. I'm using it as my main event lens until I can afford the Canon 24-70 2.8. I think they are around 200. Only issue is I am not sure if it has the manual aperture ring for the Nikon mounts.
Good luck, but my personal advice would be to keep the 24 f2 in the mix as you will greatly appreciate the faster glass.